1. Field
This disclosure is generally related to silicon deposition. More specifically, this disclosure is related to a scalable, high throughput multi-chamber batch type epitaxial reactor for silicon deposition.
2. Related Art
The negative environmental impact caused by the use of fossil fuels and their rising cost have resulted in a dire need for cleaner, cheaper alternative energy sources. Among different forms of alternative energy sources, solar power has been favored for its cleanness and wide availability.
A solar cell converts light into electricity using the photoelectric effect. There are several basic solar cell structures, including a single p-n junction, p-i-n/n-i-p, and multi-junction. A typical single p-n junction structure includes a p-type doped layer and an n-type doped layer of similar material. A hetero-junction structure includes at least two layers of materials of different bandgaps. A p-i-n/n-i-p structure includes a p-type doped layer, an n-type doped layer, and an optional intrinsic (undoped) semiconductor layer (the i-layer) sandwiched between the p-layer and the n-layer. A multi-junction structure includes multiple semiconductor layers of different bandgaps stacked on top of one another.
In a solar cell, light is absorbed near the p-n junction generating carries. The carries diffuse into the p-n junction and are separated by the built-in electric field, thus producing an electrical current across the device and external circuitry. An important metric in determining a solar cell's quality is its energy-conversion efficiency, which is defined as the ratio between power converted (from absorbed light to electrical energy) and power collected when the solar cell is connected to an electrical circuit.
Materials that can be used to construct solar cells include amorphous silicon (a-Si), polycrystalline silicon (poly-Si), crystalline-silicon (crystalline Si), cadmium telluride (CdTe), etc. FIG. 1 illustrates an exemplary crystalline-silicon thin-film solar cell. Solar cell 100 includes a low-grade crystalline-Si substrate 102, a p-type doped single-crystal Si layer 104, an n+ silicon emitter layer 106, front electrodes 108, and an Al back electrode 110. Arrows in FIG. 1 indicate incident sunlight.
Based on industrial surveys, crystalline-Si-wafer based solar cells dominate nearly 90% of the market. However, the cost of producing crystalline-Si-wafer based solar cell is high, and the waste of Si material in the processes of ingot-cutting and wafer-polishing has caused a bottleneck in the supply of crystalline-Si wafers. Due to the soaring price and the supply shortage of Si material, there has been a great interest in alternative ways to make solar cells. Recently, photovoltaic thin-film technology has been drawing vast interest because it can significantly reduce the amount of material used and thus lower the cost of solar cells. Among various competing technologies, single-crystal Si thin-film solar cells have drawn great interest for their low cost and high efficiency.
Single-crystal Si thin-film solar cells can be created using conventional semiconductor epitaxy technologies which not only reduce manufacturing costs but also enable flexible doping levels in the emitter, absorber and back surface field of the solar cell, thus enhancing its efficiency. Single-crystal Si thin-film solar cells with an efficiency as high as 17% have been demonstrated in research labs (see M. Reutuer et al., “17% Efficient 50 μm Thick Solar Cells,” Technical Digest, 17th International Photovoltaic Science and Engineering Conference, Fukuoka, Japan, p. 424).
A high-quality single-crystal Si thin film can be produced using Si epitaxy, which has been widely used in semiconductor industry to create a high-quality single-crystal Si layer for CMOS integrated circuits, power devices and high voltage discrete devices. Among possible Si epitaxial deposition techniques, trichlorosilane (TCS) based chemical-vapor-deposition (CVD) can provide a deposition rate up to 10 μm/min. Therefore, it is possible to achieve a high-throughput and low-cost epitaxial process for solar cell application.
However, there is a lack of suitable Si epitaxy tools that can meet the demand for high throughput and low deposition cost for Si film layers with thickness up to several tens of microns, as required by the solar cell industry. Existing Si epitaxy tools, such as AMC7810™ and Centura 5200™ by Applied Materials Inc. of Santa Clara, Calif., US; MT7700™ by Moore Epitaxial Inc. of Tracy, Calif., US; PE2061™ by LPE Epitaxial Technology of Italy; and Epsilon 3200™ by ASM International of the Netherlands, are optimized for the needs of semiconductor device manufacturing. Although these epitaxial tools can deliver Si films with the highest quality, these tools are not compatible, in terms of throughput and gas conversion efficiency, with the economics of the solar cell industry.
FIG. 2 presents a diagram illustrating the structure of an existing barrel epitaxial reactor, which is used for the batch process of multiple wafers. Barrel reactor 200 includes a reaction chamber 202, which has a gas inlet 204 at the top and a vent 206 at the bottom. A vertically positioned susceptor 208 holds a number of wafers, such as wafer 210. Radio frequency (RF) heating coils 212 radiate heat onto the susceptor and wafers. Although barrel reactor 200 can batch process multiple wafers, the number of wafers it can process is limited by the architect of the system, the size of the chamber, and the design of the susceptor. Once built, it is difficult to modify the reactor or the susceptor to accommodate more wafers. In addition, the susceptor needs to be rotated during deposition in order to achieve a better uniformity.
U.S. Pat. No. 6,399,510 proposed a reaction chamber that provides a bi-directional process gas flow to increase uniformity without the need for rotating susceptors. However, it does not solve the issues of low throughput, low reaction gas conversion rate, low power utilization efficiency, minimal Si deposition on the quartz chamber, and processing scalability. In addition, using the same gas lines for gas inlet and outlet increased the risk of contamination and re-deposition.